windowedMyApp/WindowedMyApp/MainWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Remoting.Contexts;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Data.Entity;

namespace WindowedMyApp
{
    /// <summary>
    /// Логика взаимодействия для MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            //ModelClass model = new ModelClass();
            //model.Klients.Where(Klients => Klients.Name == loginText.Text);

            using (ModelClass db = new ModelClass())
            {
                int HashPasswrd = passwrdText.Password.GetHashCode();
                Klients klient = db.Klients
                          .FirstOrDefault(x => x.Login == loginText.Text && x.Password == HashPasswrd);

                if (klient != null)
                {
                    ActiveWindow window = new ActiveWindow();
                    window.Show();
                    Close();
                }
                else
                {
                    MessageBox.Show("Такого пользователя не существует");
                }
            }
        }

        private void toRegisterButton_Click(object sender, RoutedEventArgs e)
        {
            RegWind wind = new RegWind();
            wind.Show();
            Close();
        }        
    }
}